Micropitting is an unexpectedly high rate of fatigue wear. It occurs in rolling sliding Elasto Hydrodynamic Lubrication (“EHL”) contact during the first few million rotation cycles of machine life. The affected gears typically have a gray matte finish on the contact surfaces with microscopic examination revealing a network of cracks and micropits 10 to 20 micrometers in diameter. This type of failure has been a chronic problem with large gearboxes including the gearboxes used in the wind turbine industry. Micropits coalesce to produce a continuous fractured surface with a characteristic dull matte appearance variously called gray staining, frosting, or, in German, graufleckigkeit when applied to gears. The related term for the phenomenon in bearings is peeling or general superficial spalling. Micropitting is generally, but not necessarily exclusively, a problem associated with heavily loaded case carburized gearing.
The progression of micropitting may eventually result in (macro)pitting, or it may progress to a point and stop. Although it may appear innocuous, such loss of metal from the gear surface causes loss of gear accuracy, increased vibration and noise, and other related problems.
Methods for measuring micropitting of gears have been developed at the FZG Institute in Munich more than a decade ago. See “Influence of the Lubricant on Pitting and Micro Pitting. Resistance of Case Carburized Gears—Test Procedures” Winter, H; Oster, P. AGMA Technical Paper 87 FTM 9, October 1987. The FZG approach was subsequently developed into a procedure sponsored by the FVA association in Germany and formally published in 1993. See “FVA-Informationsblatt Nr. 54 I-IV: Testverfahren zur Untersuchung des Schmierstoffeinflusses auf die Entstehung von Grauflecken bei Zahnradern” FVA-Nr. 54/7 Stand Juli 1993.
The FVA 54/7 procedure has become the industry standard for assessing industrial gear lubricant micropitting-resistance performance. The method uses the FZG power-circulating equipment that has two separate stages. First, a progressive loading test or stage test in which the pinion or smaller of the two gears in a set must be dismounted and rated after each 16-hour load stage from load stage 5 through load stage 10. Then the second side of the gear set is run in a stage test involving load stages 5 through 10 each 16 hours long with fresh oil. This is followed by the endurance test in which the gear is run with the same oil charge as the second stage test for a total of six 80-hour periods starting at load stage 8 for the first 80 hours, and then finishing at load stage 10 for subsequent 80 hour periods. Inspections are performed between each period. The inspections assess micropitted area of the pinion tooth flanks, pinion weight loss and the deviation of profile form. Tooth profile measurement is carried out through use of a profilometer. The sensing tip is moved from tooth tip to root and the topography is fed into a computer program. The before and after test measurements are compared and the difference reported as “profile deviation”. The damage load stage is reached when the profile deviation exceeds 7.5 μm.
Mobilgear Synthetic HydroCarbon-Xtra Micro Protection or (“SHC XMP”) sold by ExxonMobil Corporation in Fairfax Va., was commercialized in 1998 as a micropitting resistant industrial gear oil. The primary market for this lube is the wind turbine industry. Mobilgear SHC XMP was very successful in use with one exception. That exception is the superior level of performance demanded by builders today in the, Graufleckigkeit Test “GFT” FLS greater than 10 Class High. GFT Class High is a rating requiring a FLS greater than 10. Mobilgear SHC XMP 320 provides a FLS equal to 10 high.
In the last several years, there has been a number of key equipment builders in this sector that are starting to require the highest level of performance in the FVA 54 Micropitting test of FLS greater than 10. A high FLS greater than high rating require less than 7.5 microns of gear tooth profile deviation in the FVA 54 Micropitting test at the end of stage 10 loads.
In addition to micropitting, air entrainment is another issue in lubricating oils. All lubricating oil systems contain some air. It can be found in four phases: free air, dissolved air, entrained air and foam. Free air is trapped in a system, such as an air pocket in a hydraulic line. Dissolved air is in solution with the oil and is not visible to the naked eye. Foam is a collection of closely packed bubbles surrounded by thin films of oil that collect on the surface of the oil.
Air entrainment is a small amount of air in the form of extremely small bubbles (generally less than 1 mm in diameter) dispersed throughout the bulk of the oil. Agitation of lubricating oil with air in equipment, such as bearings, couplings, gears, pumps, and oil return lines, may produce a dispersion of finely divided air bubbles in the oil. If the residence time in the reservoir is too short to allow the air bubbles to rise to the oil surface, a mixture of air and oil will circulate through the lubricating oil system. This may result in an inability to maintain oil pressure (particularly with centrifugal pumps), incomplete oil films in bearings and gears, and poor hydraulic system performance or failure. Air entrainment is treated differently than foam, and is most often a completely separate problem. A partial list of potential effects of air entrainment include: pump cavitation, spongy, erratic operation of hydraulics, loss of precision control; vibrations, oil oxidation, component wear due to reduced lubricant viscosity, equipment shut down when low oil pressure switches trip, “micro-dieseling” due to ignition of the bubble sheath at the high temperatures generated by compressed air bubbles, safety problems in turbines if overspeed devices do not react quickly enough, and loss of head in centrifugal pumps.
Antifoamants, including silicone additives help produce smaller bubbles in the bulk of the oil. In stagnant systems, the combination of smaller bubbles and greater sheath density can cause serious air entrainment problems. Turbine oil systems with quiescent reservoirs of several thousand gallons may have air entrainment problems with as little as a half a part per million silicone.
Casual exposure to silicone can have a significant effect on the lubricant. There are reports of air entrainment resulting from oil passing through hoses that had been formed on a silicone-coated mandrel. In one instance in a turbine application, all sources of air were removed, and the system was carefully evaluated, component by component, to check for sources of contamination. After an exhaustive search, the culprit was found to be a silicone coating on electrical cables that were immersed in oil. Other known causes of entrainment problems include contaminants, overadditizing and reservoir design.
One widely method to test air release properties of petroleum oils is ASTM D3427-03. This test method measures the time for the entrained air content to fall to the relatively low value of 0.2% under a standardized set of test conditions and hence permits the comparison of the ability of oils to separate entrained air under conditions where a separation time is available. The significance of this test method has not been fully established. However, entrained air can cause sponginess and lack of sensitivity of the control of turbine and hydraulic systems. This test may not be suitable for ranking oils in applications where residence times are short and gas contents are high.
In the ASTM D3427 method, compressed air is blown through the test oil, which has been heated to a temperature of 25, 50, or 75° C. After the air flow is stopped, the time required for the air entrained in the oil to reduce in volume to 0.2% is usually recorded as the air release time.
Accordingly, there is a need for a lubricant that provides a consistent favorable micropitting and air release properties using high viscosities base stock blends. The present invention satisfies this need by providing a novel combination of base stocks that give the desired performance.